The anticonvulsant, lamotrigine decreases spontaneous glutamate release but increases spontaneous GABA release in the rat entorhinal cortex in vitro

Investigation of the effects of lamotrigine and clozapine in improving reversal-learning impairments induced by acute phencyclidine and D-amphetamine in the rat

RATIONALE

Phencyclidine (PCP), a glutamate/N-methyl-D-aspartate (NMDA) receptor antagonist, has been shown to induce a range of symptoms similar to those of patients with schizophrenia, while D-amphetamine induces predominantly positive symptoms. Previous studies in our laboratory have shown that PCP can selectively impair the performance of an operant reversal-learning task in the rat. Furthermore, we found that the novel antipsychotic ziprasidone, but not the classical antipsychotic haloperidol, could prevent the PCP-induced deficit.

OBJECTIVES

The aim of the present study was to validate the model further using the atypical antipsychotic clozapine and then to investigate the effects of lamotrigine, a broad-spectrum anticonvulsant that is known to reduce glutamate release in vitro and is able to prevent ketamine-induced psychotic symptoms in healthy human volunteers. A further aim was to compare effects of PCP and D-amphetamine in the test and investigate the effects of the typical antipsychotic haloperidol against the latter.

METHODS

Female hooded-Lister rats were food deprived and trained to respond for food in a reversal-learning paradigm.

RESULTS

PCP at 1.5 mg/kg and 2.0 mg/kg and D-amphetamine at 0.5 mg/kg significantly and selectively impaired performance in the reversal phase of the task. The cognitive deficit induced by 1.5 mg/kg PCP was attenuated by prior administration of lamotrigine (20 mg/kg and 30 mg/kg) or clozapine (5 mg/kg), but not haloperidol (0.05 mg/kg). In direct contrast, haloperidol (0.05 mg/kg), but not lamotrigine (25 mg/kg) or clozapine (5 mg/kg), prevented a similar cognitive impairment produced by D-amphetamine (0.5 mg/kg).

CONCLUSIONS

Our findings provide further data to support the use of PCP-induced disruption of reversal learning in rodents to investigate novel antipsychotic drugs. The results also provide evidence for different mechanisms of PCP and D-amphetamine-induced disruption of performance in the test, and their different sensitivities to typical and atypical antipsychotic drugs.

Evaluation of the effects of lamotrigine, valproate and carbamazepine in a rodent model of mania

Bipolar disorder is a psychiatric condition characterised by episodes of mania, depression, and underlying mood instability. Anticonvulsant drugs have an established place in the treatment of the disorder, but identifying novel drugs in this class is complicated by the absence of validated animal models. We have evaluated the efficacy of three anticonvulsant mood stabilising drugs (lamotrigine, valproate, and carbamazepine) in a model of mania, in which hyperactivity is induced by the combination of D-amphetamine and chlordiazepoxide. All three drugs were effective at preventing the hyperactivity. Lower doses of valproate and carbamazepine were required to prevent hyperactivity compared to doses required to block tonic-clonic seizures induced by pentylenetetrazole. Lamotrigine was equipotent in the two models. However, the complex pharmacology of the D-amphetamine/chlordiazepoxide model means that there may be several mechanisms by which hyperactivity can be reduced, and these may have more or less relevance to the treatment of bipolar disorder. To address this issue, we also evaluated effects of the three anticonvulsants on baseline locomotion, on activity in the presence of chlordiazepoxide alone, or on activity induced by D-amphetamine alone. Based on the results, we propose that hyperactivity induced by D-amphetamine/chlordiazepoxide may arise through dopaminergic drive coupled with disinhibition caused by low doses of the benzodiazepine. The efficacy of lamotrigine may then arise through a reduction in neuronal excitability or increased glutamate transmission, these latter a consequence of the disinhibition. Carbamazepine may also reduce excitability and glutamate release, but its broader pharmacology, manifested by sedation at higher doses complicates interpretation of its efficacy and reflects its poorer tolerability in the clinic. Valproate may be effective, at least in part, through an enhancement of GABAergic transmission. The predictive validity of the D-amphetamine/chlordiazepoxide model for efficacy in bipolar disorder remains to be established, and research with a wider range of clinically tested drugs is warranted to help validate the model further. In the meantime, the model may be useful for distinguishing novel anticonvulsant drugs with different mechanisms of action.

Pharmacological strategies in relieving neuropathic pain

The pharmacological treatment of neuropathic pain relies, to a large extent, on drugs belonging to a small number of defined classes. Opioids, tricyclic antidepressants, antiepileptic drugs and membrane stabilisers form the current basis of treatment. Varying levels of evidence support the use of individual members of these classes and overall show no indication that one class of drug, or individual drug has universal effectiveness. More refined knowledge of the modes of action of these agents used to treat neuropathic pain should lead to a more logical approach to the management of this difficult series of conditions. A number of drugs currently licensed for a different indication have recently had an analgesic effect in neuropathic pain attributed to them. In addition, a number of novel compounds are undergoing investigation and provide hope of dicovering more efficacious treatment options in the future.

Effects of combined lamotrigine and valproate on basal and stimulated extracellular amino acids and monoamines in the hippocampus of freely moving rats

The antiepileptic drugs sodium valproate (VPA) and lamotrigine (LTG) are increasingly used in combination in patients in whom monotherapy has failed to control seizures. Although these drugs are known to interact pharmacokinetically, several authors have proposed a pharmacodynamic interaction between the two. In order to investigate this we have studied the effects of combined treatment with LTG and VPA on basal and stimulated extracellular aspartate (ASP), glutamate (GLU), taurine (TAU), gamma amino butyric acid (GABA), 5-hydroxytryptamine (5-HT) and dopamine (DA) release in the hippocampus of freely moving rats using microdialysis. Additionally, we measured the possible effect of VPA on LTG in plasma, whole brain and dialysates. Neither LTG (10 mg/kg) nor VPA (300 mg/kg) given alone significantly altered basal levels of ASP, GLU or TAU. When given together, however, the two drugs significantly reduced extracellular ASP and GLU while increasing TAU levels. In the case of GABA, LTG was without effect on basal levels of the transmitter, but these increased following VPA and this persisted with both drugs. When transmitter release was stimulated by 50 muM veratridine, marked increases in the release of all amino acids occurred and this was decreased by LTG in all cases. VPA alone only altered GABA release, increasing it by approximately the same extent as basal GABA. For all of the amino acids studied, however, VPA reversed the decreases in release seen after LTG. VPA and LTG increased and decreased respectively basal 5-HT and DA. When given together the increase in extracellular 5-HT was greatly prolonged, but no effect on DA release was seen. When 5-HT release was evoked by veratridine this was increased by VPA and no other treatment. With DA, however, neither drug alone altered evoked release, but the two combined led to a marked increase. Co-administration of VPA with LTG showed no significant effect of this combination on LTG in any of the three compartments studied indicating that in this case a significant pharmacokinetic contribution to our findings is unlikely, which suggests that there is a probable pharmacodynamic interaction of the two drugs.

Effects of acute and chronic lamotrigine treatment on basal and stimulated extracellular amino acids in the hippocampus of freely moving rats

The antiepileptic drug lamotrigine (LTG) is a relatively novel anticonvulsant frequently used in polytherapy and increasingly in monotherapy. LTG is believed to act by reducing excitatory glutamate (GLU) release due to an inhibition of Na(+) channels. In the present study, we have investigated the effects of acute and chronic (up to 21 days) treatment with LTG on basal and either veratridine- or KCl-stimulated release of aspartate (ASP), GLU, taurine (TAU) and GABA in the hippocampus of freely moving rats using microdialysis. Additionally, we have measured LTG concentrations in the plasma, whole brain and extracellular fluid of rats at the same time points. LTG significantly reduced basal ASP and GLU but only at the highest dose used (20 mg/kg) and was entirely without effect on basal TAU or GABA. When either veratridine or 100 mM KCl were added to the infusion medium amino acid release was evoked although the extent of this varied from one amino acid to another. LTG (10 mg/kg) reduced veratridine-evoked release of all four amino acids studied, although this was most marked in the case of GLU. LTG had no effect on KCl-stimulated amino acid release. When given for up to 21 days (2 x 5 mg/kg/day), LTG had no effect on basal amino acid levels. In contrast, LTG demonstrated over the time period studied an increasingly inhibitory effect on veratridine-evoked amino acid release. This effect of the drug was proportionally much greater in the case of GLU than for the other three amino acids studied. Measurement of plasma, whole brain tissue and extracellular LTG showed that in each of these compartments, it had reached an apparent steady state within 4 days of commencement of treatment and appeared to mirror the neurochemical changes measured. Our estimate of plasma LTG indicates that during chronic study, this was well within the therapeutic range, suggesting that the current neurochemical observations are clinically relevant.

Dual effects of gabapentin and pregabalin on glutamate release at rat entorhinal synapses in vitro

We have recently shown that the anticonvulsant drugs phenytoin, lamotrigine and sodium valproate all reduce the release of glutamate at synapses in the entorhinal cortex in vitro. In the present investigation we determined whether this property was shared by gabapentin and pregabalin, using whole-cell patch-clamp recordings of excitatory postsynaptic currents (EPSCs) in layer V neurons in slices of rat entorhinal cortex. Both drugs reduced the amplitude and increased the paired-pulse ratio of EPSCs evoked by electrical stimulation of afferent inputs, suggesting a presynaptic effect to reduce glutamate release. The frequency of spontaneous EPSCs (sEPSCs) was concurrently reduced by GBP, further supporting a presynaptic action. There was no significant change in amplitude although a slight reduction was seen, particularly with gabapentin, which may reflect a reduction in the number of larger amplitude sEPSCs. When activity-independent miniature EPSCs were recorded in the presence of tetrodotoxin, both drugs continued to reduce the frequency of events with no change in amplitude. The reduction in frequency induced by gabapentin or pregabalin was blocked by application of the l-amino acid transporter substrate l-isoleucine. The results show that gabapentin and pregabalin, like other anticonvulsants, reduce glutamate release at cortical synapses. It is possible that this reduction is a combination of two effects: a reduction of activity-dependent release possibly via interaction with P/Q-type voltage-gated Ca channels, and a second action, as yet unidentified, occurring downstream of Ca influx into the presynaptic terminals.

Interleaved transcranial magnetic stimulation/functional MRI confirms that lamotrigine inhibits cortical excitability in healthy young men

Little is known about how lamotrigine (LTG) works within brain circuits to achieve its clinical effects. We wished to determine whether the new technique of interleaved transcranial magnetic stimulation (TMS)/functional magnetic resonance imaging (fMRI) could be used to assess the effects of LTG on activated motor or prefrontal/limbic circuits. We carried out a randomized, double-blind, crossover trial involving two visits 1 week apart with TMS measures of cortical excitability and blood oxygen level-dependent TMS/fMRI. Subjects received either a single oral dose of 325 mg of LTG or placebo on each visit. In all, 10 subjects provided a complete data set that included interleaved TMS/fMRI measures and resting motor threshold (rMT) determinations under both placebo and LTG conditions. A further two subjects provided only rMT data under the two drug conditions. LTG caused a 14.9+/-9.6% (mean+/-SD) increase in rMT 3 h after the drug, compared with a 0.6+/-10.9% increase 3 h after placebo (t=3.41, df =11, p<0.01). fMRI scans showed that LTG diffusely inhibited cortical activation induced by TMS applied over the motor cortex. In contrast, when TMS was applied over the prefrontal cortex, LTG increased the TMS-induced activation of limbic regions, notably the orbitofrontal cortex and hippocampus. These results suggest that LTG, at clinically relevant serum concentrations, has a general inhibitory effect on cortical neuronal excitability, but may have a more complex effect on limbic circuits. Furthermore, the interleaved TMS/fMRI technique may be a useful tool for investigating regional brain effects of psychoactive compounds.

Disruption of prepulse inhibition in mice lacking mGluR1

Sensorimotor gating, measured by prepulse inhibition of the startle response (PPI), is a cross-species form of information processing that is deficient in patients with schizophrenia and is widely used as a model to study the neurobiology of this disorder. The eight known metabotropic glutamate receptors (mGluRs) are divided into three groups on the basis of sequence homology and pharmacological properties. Group I consists of mGluR5 and mGluR1, both of which are coupled positively to phospholipase C. Mice lacking mGluR5 exhibit a deficit in PPI. Like mGluR5, mGluR1 is located in regions that are involved in the modulation of PPI. To test the hypothesis that mGluR1 is involved in the modulation of PPI we assessed PPI in mGluR1 knockout (KO) mice. Littermate mGluR1 wild-type and KO mice were tested at multiple ages in a standard PPI paradigm containing a 65 dB background, 120 dB pulses and prepulses of 69, 73 and 77 dB. At all ages tested, mGluR1 KO mice exhibited a significant PPI deficit. The PPI deficit of the mGluR1 KO mice was not further exaggerated by administration of the N-methyl-d-aspartate antagonist phencyclidine nor was it reversed by administration of the dopamine antagonist raclopride (3.0 mg/kg). The PPI deficit of the mGluR1 KO mice was, however, ameliorated by administration of the mood stabilizer lamotrigine (27 mg/kg base equivalent weight), though increases in PPI were also seen with lamotrigine in the wild-type mice. Thus, both group I metabotropic glutamate receptors are involved in the regulation of PPI in mice.

Pharmacological management of neuropathic pain

In health, the nervous system exists in a balance between inhibitory and excitatory influences. This balance may be upset if neural tissue is damaged or irritated and may give rise to neuropathic pain. Such neuropathic pain does not respond consistently to opioid analgesics or NSAIDs and it may therefore be necessary to utilise other therapeutic agents with known activity on either the excitatory or inhibitory components of the pain pathway. These other agents are traditionally considered with reference to their original uses; we still refer to tricyclic antidepressants (TCAs) and anticonvulsant drugs when a consideration of their modes of action may allow more rational use. For example, carbamazepine is related to the TCAs by virtue of its chemical structure and proposed mode of action and yet is still classified as an anticonvulsant drug. With respect to the opioids, increasing evidence points to an analgesic effect in neuropathic pain, although concerns regarding tolerance and dependence still prevent more widespread use. The anticonvulsants comprise a group of compounds possessing anticonvulsant and analgesic properties, but each possesses differing modes of action and so several members of the class should be tried before a conclusion is reached that they, as a whole, are ineffective. TCAs may also have a role in the treatment of neuropathic pain. As with all drugs, if their use is not associated with pain relief in a defined period of time, their use should be terminated. Topical TCAs may also have a role where the area of neuropathic pain is small. Other options, such as SSRIs, membrane stabilisers, capsaicin, baclofen and clonidine may have potential in treating neuropathic pain. The available evidence regarding the efficacy of currently available agents for the treatment of neuropathic pain is sparse. With the knowledge of achieving analgesia, according to the modes of actions of various agents it is hoped that the treatment of this difficult condition may be more logical and successful.

Does SPM 927 have an analgesic effect in human neuropathic pain? An open label study

The aim of the study is to establish if the putative anticonvulsant SPM 927 has an analgesic effect in human neuropathic pain and to assess its tolerability. This is an open label study of 25 adult human subjects with resistant neuropathic pain. Subjects were treated with SPM 927 in a dose-escalating scheme to 600 mg daily, if tolerated. Treatment was continued for 4 weeks then withdrawn without tapering. Pain scores were recorded using a 11-point Likert score and a categorical pain-rating scale. Laboratory parameters and, electrocardiographs (ECGs) were collected; side effects were noted. Of the 25 enrolled subjects, 12 completed the study according to the protocol. The remaining subjects dropped out due to adverse events (n=12) or withdrawn consent. Mean daily pain scores (Likert score) fell by 0.83 (95% CI -1.77, +0.11) at the end of maintenance and rose by 0.58 (95% CI -0.23, +1.40) after withdrawal of SPM 927. Similar changes were seen in the categorical pain-rating scores. There were decreases in the mean scores for shooting pain, paraesthesia, and allodynia, but much less change in the numbness and burning-pain scores. The most common side effects were nausea, dizziness, leukocytosis, and increased ALT. No consistent changes in ECG recordings or haemodynamic variables were observed. SPM 927 may have an analgesic effect in human neuropathic pain and was reasonably well tolerated in this study. These data support the continued clinical development of SPM 927 for neuropathic pain.

Glutamate-dependent regulation of cholinergic phenotype in hypothalamic neurons

Glutamate NMDA receptor antagonists are used clinically. However, they have serious side effects, some of which are presumably due to an increase in acetylcholine transmission. Our previous experiments revealed acetylcholine-dependent excitation in rat hypothalamic cultures after a chronic glutamate receptor blockade. Dextromethorphan, amantadine, and eliprodil are NMDA receptor antagonists. Lamotrigine inhibits synaptic glutamate release. These drugs are used clinically. Here, using calcium imaging and immunocytochemistry, we demonstrate that a chronic treatment with each of these drugs induced acetylcholine activity and choline acetyltransferase immunoreactivity in rat hypothalamic (but not cortical) cultures. These data support the possibility that some side effects of anti-glutamate drugs in vivo may be due to the increase in cholinergic properties in certain regions of the CNS.

GABAergic dysfunction in mood disorders

The authors review the available literature on the preclinical and clinical studies involving GABAergic neurotransmission in mood disorders. Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter present almost exclusively in the central nervous system (CNS), distributed across almost all brain regions, and expressed in interneurons modulating local circuits. The role of GABAergic dysfunction in mood disorders was first proposed 20 years ago. Preclinical studies have suggested that GABA levels may be decreased in animal models of depression, and clinical studies reported low plasma and CSF GABA levels in mood disorder patients. Also, antidepressants, mood stabilizers, electroconvulsive therapy, and GABA agonists have been shown to reverse the depression-like behavior in animal models and to be effective in unipolar and bipolar patients by increasing brain GABAergic activity. The hypothesis of reduced GABAergic activity in mood disorders may complement the monoaminergic and serotonergic theories, proposing that the balance between multiple neurotransmitter systems may be altered in these disorders. However, low GABAergic cortical function may probably be a feature of a subset of mood disorder patients, representing a genetic susceptibility. In this paper, we discuss the status of GABAergic hypothesis of mood disorders and suggest possible directions for future preclinical and clinical research in this area.

Interactions of lamotrigine with topiramate and first-generation antiepileptic drugs in the maximal electroshock test in mice: an isobolographic analysis

PURPOSE

The study investigated the types of interactions between lamotrigine (LTG) and first-generation antiepileptic drugs (AEDs) or topiramate (TPM) with isobolographic analysis.

METHODS

Anticonvulsant and adverse-effect profiles of combinations of LTG with other AEDs, at fixed ratios of 1:3, 1:1, and 3:1, were evaluated in the maximal electroshock (MES)-induced seizures and the chimney test (motor performance) in mice, which allowed the determination of benefit indices (BIs) for individual combinations.

RESULTS

Combinations of LTG with TPM or valproate (VPA), at fixed ratios of 1:1, were significantly supraadditive (synergistic) in the MES test and, simultaneously, subadditive (antagonistic) in the chimney test, showing the best profile for AED combinations. In contrast, combinations between LTG and carbamazepine (CBZ), in terms of antiseizure protection against MES, were subadditive (antagonistic) and additive in the chimney test, resulting in unfavorable AED combinations. Moreover, the combination of LTG with phenobarbital (PB), at a fixed ratio of 1:1, despite synergy in the MES test, also was synergistic in the chimney test, resulting in a modest BI for AED combination. LTG combined with phenytoin was additive in both the MES and chimney tests in mice. The remaining combinations, at fixed ratios not mentioned earlier, also showed an average BI for AED combinations. Furthermore, LTG combined with all studied AEDs did not affect long-term memory in mice. None of the AEDs influenced the free plasma level of LTG, whereas LTG slightly reduced the free plasma concentration of PB.

CONCLUSIONS

Interactions between LTG and TPM or LTG and VPA at a fixed ratio of 1:1 might be profitable from a preclinical point of view, displaying the most optimal BI.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Effect of lamotrigine on plasma GABA levels in healthy humans

Lamotrigine, a new anticonvulsant, has been reported to be useful in treating bipolar depression, rapid cycling, and other phases of bipolar disorder. However, the mechanism of action underlying its efficacy in mood disorders is still not known. Since there is evidence for gamma-aminobutyric acid (GABA) involvement in the pathophysiology and treatment response of patients with bipolar disorder, this study was designed to examine the effect of lamotrigine on plasma GABA levels in healthy humans. Eleven healthy volunteers with no lifetime history of psychiatric illness or family history in first-degree relatives were recruited. Each subject received lamotrigine 100 mg/day for 1 week. Blood samples for assay of plasma levels of GABA were taken from each subject before and after administration of lamotrigine. Plasma GABA levels were analyzed using high-pressure liquid chromatography (HPLC) with fluorescence detection after derivatization with o-phthaldialdehyde (OPA). We found no significant difference in the plasma GABA levels of the study subjects before and after treatment with lamotrigine. The finding of this study suggests that lamotrigine in the dose used in this study does not appear to enhance GABA levels in humans.

Coadministration of gabapentin or MK-801 with lamotrigine slows tolerance to its anticonvulsant effects on kindled seizures

The development of tolerance to therapeutic effects of antiepileptic drugs can be a problem in the treatment of epilepsy, bipolar disorder, and pain syndromes. In the present study, acute treatment with the new antiepileptic drug lamotrigine (LTG, 15 mg/kg) markedly suppressed seizure stage and seizure duration in amygdala-kindled rats; but this antiseizure effect was rapidly lost following 4-8 days of repeated treatment. When gabapentin (GBP, 20 mg/kg) was coadministered with LTG, the ability of LTG to suppress seizure stage, seizure duration, and after-discharge (AD) duration was markedly extended. In addition, GBP coadministration with LTG decreased the number of animals that developed LTG-related running fits (Stage 6 seizures) and lengthened the number of days required to develop running fits or complete tolerance. Neither acute nor repeated treatment with MK-801 (0.3 mg/kg), a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, had effects on kindled seizures. However, cotreatment with MK-801 markedly extended the anticonvulsant effects of LTG on the three seizure indices and reduced running fits. These data indicate that cotreatment with either GBP or MK-801 slows tolerance development to the anticonvulsant effects of LTG on kindled seizures. Therapeutic implications of the present study remain to be explored.

Influence of administration vehicles and drug formulations on the pharmacokinetic profile of lamotrigine in rats

Given that administration vehicles and drug formulations can affect drug bioavailability, their influence on the pharmacokinetic profile of lamotrigine (LTG), a new-generation anti-epileptic drug, was studied in rats. Three different formulations administered intraperitoneally at a dose of 10 mg/kg were used: (1) LTG suspended in a 0.25% methylcelulose solution, (2) LTG dissolved in a 50% propylene glycol solution, and (3) LTG isethionate dissolved in distilled water. Plasma and brain homogenate levels were determined in order to evaluate vehicle-dependent drug absorption. The results demonstrated rapid absorption of LTG when it was administered as an aqueous solution, in contrast to a slower and more erratic absorption after the injection of either the lipophilic solution or the suspension. A plasma peak was achieved 15 min post-dose with the aqueous solution, with a brain peak being achieved 15 min later, while with the other formulations both plasma and brain homogenate peaks were reached 2 h after LTG administration. This study suggests that LTG isethionate dissolved in distilled water is the most suitable formulation for successful LTG pharmacokinetic studies in rats.

Glutamate and GABA systems as targets for novel antidepressant and mood-stabilizing treatments

Glutamate and gamma-amino butyric acid (GABA) systems are emerging as targets for development of medications for mood disorders. There is increasing preclinical and clinical evidence that antidepressant drugs directly or indirectly reduce N-methyl-D-aspartate glutamate receptor function. Drugs that reduce glutamatergic activity or glutamate receptor-related signal transduction may also have antimanic effects. Recent studies employing magnetic resonance spectroscopy also suggest that unipolar, but not bipolar, depression is associated with reductions in cortical GABA levels. Antidepressant and mood-stabilizing treatments also appear to raise cortical GABA levels and to ameliorate GABA deficits in patients with mood disorders. The preponderance of available evidence suggests that glutamatergic and GABAergic modulation may be an important property of available antidepressant and mood-stabilizing agents. Future research will be needed to develop and evaluate new agents with specific glutamate and GABA receptor targets in the treatment of mood disorders.

Lamotrigine update and its use in mood disorders

OBJECTIVE

To provide a qualitative, systematic update and review of the pharmacology, pharmacokinetics, efficacy in mood disorders, adverse effects, and costs of lamotrigine.

DATA SOURCES

Citations obtained from MEDLINE searches (1985-September 2001) using lamotrigine as a text word, articles identified in reference lists of pertinent articles, abstracts presented at conferences, and research data from GlaxoSmithKline.

DATA EXTRACTION

English-language articles were considered for possible inclusion. Each title and abstract was examined to determine whether the publication contained up-to-date information relevant to the objective. Twenty clinical trials that provided data on response rates in mood disorders were tabulated.

DATA SYNTHESIS

Lamotrigine's primary action is to modulate voltage-gated sodium channels. Evidence suggests that it decreases glutamate transmission, directly reduces calcium influx, mildly blocks transmitter reuptake, and alters intracellular mechanisms of resting transmitter release. The average half-life of lamotrigine is approximately 24 hours, but decreases to approximately 7.4 hours when used concurrently with phenytoin, and increases to approximately 59 hours with valproic acid. Seven of the 20 clinical trials were randomized, double-blind, and controlled. Existing data are inadequate to evaluate lamotrigine use in major depression. The pooled response rates for patients with depressed, manic, mixed, and rapid cycling bipolar disorder were similar, ranging from 52% to 63%. Adverse effects are infrequent when the drug is used alone, but become more frequent when lamotrigine is combined with other anticonvulsants. While most rashes are mild, approximately 1 in 500 patients develops exfoliative dermatitis. A slow upward dose titration is recommended to reduce the incidence of serious rash, but this may delay the attainment of adequate dosage for 6 weeks. Lamotrigine has positive effects on cognitive function, but occasionally produces insomnia. Lamotrigine costs 2-4 times more than lithium, carbamazepine, and generic valproic acid.

CONCLUSIONS

When efficacy, adverse effects, and cost are considered, lamotrigine should probably be reserved as a second-line agent for bipolar depression.

Dendrotoxin sensitive potassium channels modulate GABA but not glutamate release in the rat entorhinal cortex in vitro

We have previously shown that the anticonvulsant drug, phenytoin, increases the frequency and amplitude of spontaneous inhibitory postsynaptic currents at GABA synapses on principal neurones in the rat entorhinal cortex. This effect is similar to that seen at other GABA synapses following blockade of voltage-gated potassium channels (Kv1.1, 1.2 and 1.6) with alpha-dendrotoxin. In the present study we examined whether dendrotoxins can alter GABA release at synapses in the entorhinal cortex. We recorded spontaneous inhibitory postsynaptic currents using whole cell voltage clamp techniques in slices of rat entorhinal cortex in vitro. alpha-Dendrotoxin evoked an increase in frequency and amplitude of spontaneous inhibitory postsynaptic currents, an effect that was blocked by prior perfusion with tetrodotoxin. The effect of the toxin did not occlude the increase in spontaneous inhibitory postsynaptic currents seen with phenytoin. Indeed, the effect of the two drugs together was, at least, additive on GABA release. Perfusion with the specific Kv1.1 blocker, dendrotoxin-K had no effect on GABA release. In addition, alpha-dendrotoxin had no effect on frequency or amplitude of spontaneous excitatory postsynaptic currents at glutamate synapses on entorhinal cortex neurones. We conclude that K-channels containing the Kv1.2 and/or 1.6 subunits modulate the release of GABA, but not glutamate in the entorhinal cortex. The modulation of GABA release by phenytoin is unlikely to be due to an effect on these channels.

Lamotrigine reduces spontaneous and evoked GABAA receptor-mediated synaptic transmission in the basolateral amygdala: implications for its effects in seizure and affective disorders

Lamotrigine (LTG) is an antiepileptic drug that is also effective in the treatment of certain psychiatric disorders. Its anticonvulsant action has been attributed to its ability to block voltage-gated Na(+) channels and reduce glutamate release. LTG also affects GABA-mediated synaptic transmission, but there are conflicting reports as to whether inhibitory transmission is enhanced or suppressed by LTG. We examined the effects of LTG on GABA(A) receptor-mediated synaptic transmission in slices from rat amygdala, a brain area that is particularly important in epileptogenesis and affective disorders. In intracellular recordings, LTG (100 microM) reduced GABA(A) receptor-mediated IPSPs evoked by electrical stimulation in neurons of the basolateral nucleus. In whole-cell recordings, LTG (10, 50 and 100 microM) decreased the frequency and amplitude of spontaneous IPSCs, as well as the amplitude of evoked IPSCs, but had no effect on the kinetics of these currents. LTG also had no effects on the frequency, amplitude or kinetics of miniature IPSCs recorded in the presence of TTX. These results suggest that in the basolateral amygdala, LTG suppresses GABA(A) receptor-mediated synaptic transmission by a direct and/or indirect effect on presynaptic Ca(++) influx. The modulation of inhibitory synaptic transmission may be an important mechanism underlying the psychotropic effects of LTG.

Lamotrigine

Lamotrigine is a newly available antiepileptic drug with a broad spectrum of efficacy and a tolerability profile that is favorable for use in many patients. Problems with rash are related to rate of dose ascension and drug interactions with valproate. Efficacy in treatment of mood disorder and some pain syndromes will broaden the use of this unique compound.

Lamotrigine therapy of epilepsy in tuberous sclerosis

PURPOSE

Lamotrigine (LTG), a newer antiepileptic drug (AED), has activity against both partial-onset and generalized seizures. Its reported benefits for behavior, and its effectiveness in Lennox-Gastaut syndrome and other forms of refractory epilepsy, make it a logical choice for treatment of epilepsy in tuberous sclerosis complex (TSC). We present our experience with LTG therapy of epilepsy in 57 patients with TSC.

METHODS

Patients fulfilled the diagnostic criteria for clinically definite TSC. LTG was initiated and increased until improvement in seizure frequency was noted, intolerable side effects occurred, or maximal doses were reached. Seizure frequency and behavioral changes were recorded during LTG therapy and compared with those prior to the introduction of LTG.

RESULTS

Twenty-four (42%) were seizure free, and 21 (37%) had a >50% reduction in seizure frequency. Eighteen (32%) had subjectively improved behavior and/or alertness with daily activities. Thirty-eight (67%) had no change in this regard, whereas one (2%) became worse. Responders were more likely to not have a history of infantile spasms, and to have experienced only partial seizures (p < 0.05). Otherwise no phenotypic correlations with response were apparent.

CONCLUSIONS

Among patients with TSC and epilepsy, LTG was effective and well tolerated, including as initial monotherapy. Improved alertness and behavior were apparent in many patients. The incidence of side effects is similar to that reported for other pediatric populations with symptomatic partial epilepsy. The usefulness of LTG in TSC may relate to an underlying defect of glutamatergic neurotransmission in partial epilepsy.